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Abstract
A mesoscale numerical simulation (35 km) of a return-flow event over the Gulf of Mexico that occurred during the Gulf of Mexico Experiment (GUFMEX) is presented in order to examine the structure and the transformation of the polar air mass and to assess the model's skill in simulating the event. The study deals with the phase of cold-air outbreak over the Gulf of Mexico and the subsequent rapid modification of the cold air mass by the underlying warm ocean, prior to the onset or return flow.
The investigation focuses on the physical processes operating during the airmass transformation, notably the air-sea fluxes and the vertical destabilization of the airman. The results are compared with various data gathered during GUFMEX and suggest that a realistic simulation of airmass transformation can be obtained. The results indicate a strong interplay between 1) large-scale subsidence above the planetary boundary layer behind the front and 2) destabilization near the sea surface and in the boundary layer. In particular, advective processes play a central role in the airmass modification above the boundary layer and in the maintenance of a strong capping inversion. However, very large surface energy fluxes and vigorous turbulent vertical mixing appear as dominant mechanisms within the boundary layer itself. A sensitivity experiment where surface energy fluxes are turned off supports these conclusions and clearly demonstrates their impact on the advance of the cold air mass over the Gulf and on the changes in moisture and stability of the return flow.
Abstract
A mesoscale numerical simulation (35 km) of a return-flow event over the Gulf of Mexico that occurred during the Gulf of Mexico Experiment (GUFMEX) is presented in order to examine the structure and the transformation of the polar air mass and to assess the model's skill in simulating the event. The study deals with the phase of cold-air outbreak over the Gulf of Mexico and the subsequent rapid modification of the cold air mass by the underlying warm ocean, prior to the onset or return flow.
The investigation focuses on the physical processes operating during the airmass transformation, notably the air-sea fluxes and the vertical destabilization of the airman. The results are compared with various data gathered during GUFMEX and suggest that a realistic simulation of airmass transformation can be obtained. The results indicate a strong interplay between 1) large-scale subsidence above the planetary boundary layer behind the front and 2) destabilization near the sea surface and in the boundary layer. In particular, advective processes play a central role in the airmass modification above the boundary layer and in the maintenance of a strong capping inversion. However, very large surface energy fluxes and vigorous turbulent vertical mixing appear as dominant mechanisms within the boundary layer itself. A sensitivity experiment where surface energy fluxes are turned off supports these conclusions and clearly demonstrates their impact on the advance of the cold air mass over the Gulf and on the changes in moisture and stability of the return flow.
Abstract
The relative roles of implicit and explicit condensation schemes in the numerical representation of a squall line that occurred on 7–8 May 1995 over the southern Great Plains are examined in this study using Mesoscale Compressible Community model integrations at 2-, 6-, 18-, and 50-km resolution. Results from the 2-km model in which condensation is explicitly represented agree best with observations and are used as “synthetic” data to evaluate the performance of lower-resolution configurations.
It is found that the representation of the squall system greatly deteriorates as resolution is decreased and that the relative roles of the implicit and explicit condensation schemes change dramatically. At 6-km resolution, the leading convective band is barely resolved by the model, and the implicit–explicit partition of precipitation is ambiguous because both implicit and explicit schemes are active simultaneously at the leading edge of the system. In spite of this ambiguity, it is found that use of a deep convection scheme is still beneficial to the squall-line simulation. At 18 km, the convective line is not resolved by the model, and its effect is completely due to the implicit scheme. The mesoscale circulations in the trailing anvil region of the squall system are generated at the small end of the model resolvable scales and are exaggeratedly intense. There is no ambiguity concerning the partition of condensation into implicit and explicit components at this resolution, but the relative intensity of precipitation produced by the two cloud schemes is opposite to what is observed, considering that the implicit scheme is supposed to represent subgrid-scale convection at the leading edge of the system, and the explicit scheme the grid-scale condensation in the trailing anvil. At 50 km, both the leading convection and the mesoscale circulations in the trailing anvil have to be parameterized because they are not resolved at the model grid scale. The precipitation and internal structures associated with the squall line are thus not well represented at this resolution.
The results also show that all the configurations produce precipitation accumulations that are much larger than observations. This problem is most important at 18-km resolution. Grid-scale condensation is mostly responsible for this rainfall overestimation. It is suggested that this problem is linked to a misrepresentation of convective-scale processes.
Abstract
The relative roles of implicit and explicit condensation schemes in the numerical representation of a squall line that occurred on 7–8 May 1995 over the southern Great Plains are examined in this study using Mesoscale Compressible Community model integrations at 2-, 6-, 18-, and 50-km resolution. Results from the 2-km model in which condensation is explicitly represented agree best with observations and are used as “synthetic” data to evaluate the performance of lower-resolution configurations.
It is found that the representation of the squall system greatly deteriorates as resolution is decreased and that the relative roles of the implicit and explicit condensation schemes change dramatically. At 6-km resolution, the leading convective band is barely resolved by the model, and the implicit–explicit partition of precipitation is ambiguous because both implicit and explicit schemes are active simultaneously at the leading edge of the system. In spite of this ambiguity, it is found that use of a deep convection scheme is still beneficial to the squall-line simulation. At 18 km, the convective line is not resolved by the model, and its effect is completely due to the implicit scheme. The mesoscale circulations in the trailing anvil region of the squall system are generated at the small end of the model resolvable scales and are exaggeratedly intense. There is no ambiguity concerning the partition of condensation into implicit and explicit components at this resolution, but the relative intensity of precipitation produced by the two cloud schemes is opposite to what is observed, considering that the implicit scheme is supposed to represent subgrid-scale convection at the leading edge of the system, and the explicit scheme the grid-scale condensation in the trailing anvil. At 50 km, both the leading convection and the mesoscale circulations in the trailing anvil have to be parameterized because they are not resolved at the model grid scale. The precipitation and internal structures associated with the squall line are thus not well represented at this resolution.
The results also show that all the configurations produce precipitation accumulations that are much larger than observations. This problem is most important at 18-km resolution. Grid-scale condensation is mostly responsible for this rainfall overestimation. It is suggested that this problem is linked to a misrepresentation of convective-scale processes.
Abstract
The new Recherche Prévision Numérique (NEW-RPN) model, a coupled system including a multilayer snow thermal model (SNTHERM) and the sea ice model currently used in the Meteorological Service of Canada (MSC) operational forecasting system, was evaluated in a one-dimensional mode using meteorological observations from the Surface Heat Budget of the Arctic Ocean (SHEBA)’s Pittsburgh site in the Arctic Ocean collected during 1997/98. Two parameters simulated by NEW-RPN (i.e., snow depth and ice thickness) are compared with SHEBA’s observations and with simulations from RPN, MSC’s current coupled system (the same sea ice model and a single-layer snow model). Results show that NEW-RPN exhibits better agreement for the timing of snow depletion and for ice thickness. The profiles of snow thermal conductivity in NEW-RPN show considerable variability across the snow layers, but the mean value (0.39 W m−1 K−1) is within the range of reported observations for SHEBA. This value is larger than 0.31 W m−1 K−1, which is commonly used in single-layer snow models. Of particular interest in NEW-RPN’s simulation is the strong temperature stratification of the snowpack, which indicates that a multilayer snow model is needed in the SHEBA scenario. A sensitivity analysis indicates that snow compaction is also a crucial process for a realistic representation of the snowpack within the snow/sea ice system. NEW-RPN’s overestimation of snow depth may be related to other processes not included in the study, such as small-scale horizontal variability of snow depth and blowing snow processes.
Abstract
The new Recherche Prévision Numérique (NEW-RPN) model, a coupled system including a multilayer snow thermal model (SNTHERM) and the sea ice model currently used in the Meteorological Service of Canada (MSC) operational forecasting system, was evaluated in a one-dimensional mode using meteorological observations from the Surface Heat Budget of the Arctic Ocean (SHEBA)’s Pittsburgh site in the Arctic Ocean collected during 1997/98. Two parameters simulated by NEW-RPN (i.e., snow depth and ice thickness) are compared with SHEBA’s observations and with simulations from RPN, MSC’s current coupled system (the same sea ice model and a single-layer snow model). Results show that NEW-RPN exhibits better agreement for the timing of snow depletion and for ice thickness. The profiles of snow thermal conductivity in NEW-RPN show considerable variability across the snow layers, but the mean value (0.39 W m−1 K−1) is within the range of reported observations for SHEBA. This value is larger than 0.31 W m−1 K−1, which is commonly used in single-layer snow models. Of particular interest in NEW-RPN’s simulation is the strong temperature stratification of the snowpack, which indicates that a multilayer snow model is needed in the SHEBA scenario. A sensitivity analysis indicates that snow compaction is also a crucial process for a realistic representation of the snowpack within the snow/sea ice system. NEW-RPN’s overestimation of snow depth may be related to other processes not included in the study, such as small-scale horizontal variability of snow depth and blowing snow processes.
Abstract
In an effort to improve operational forecasts of mesoscale convective systems (MCSs), a mesoscale version of the operational Canadian Regional Finite-Element (RFE) Model with a grid size of 25 km is used to predict an intense MCS that occurred during 10–11 June 1985. The mesoscale version of the RFE model contains the Fritsch–Chappell scheme for the treatment of subgrid-scale convective processes and an explicit scheme for the treatment of grid-scale cloud water (ice) and rainwater (snow).
With higher resolution and improved condensation physics, the RFE model reproduces many detailed structures of the MCS, as compared with all available observations. In particular, the model predicts well the timing and location of the leading convective line followed by stratiform precipitation; the distribution of surface temperature and pressure perturbations (e.g., cold outflow boundaries, mesolows, mesohighs, and wake lows); and the circulation of front-to-rear flows at both upper and lower levels separated by a rear-to-front flow at midlevels.
Several sensitivity experiments are performed to examine the effects of varying initial conditions and model physics on the prediction of the squall system. It is found that both the moist convective adjustment and the Kuo schemes can reproduce the line structure of convective precipitation. However, these two schemes are unable to reproduce the internal flow structure of the squall system and the pertinent surface pressure and thermal perturbations. It is emphasized that as the grid resolution increases, reasonable treatments of both parameterized and grid-scale condensation processes are essential in obtaining realistic predictions of MCSs and associated quantitative precipitation.
Abstract
In an effort to improve operational forecasts of mesoscale convective systems (MCSs), a mesoscale version of the operational Canadian Regional Finite-Element (RFE) Model with a grid size of 25 km is used to predict an intense MCS that occurred during 10–11 June 1985. The mesoscale version of the RFE model contains the Fritsch–Chappell scheme for the treatment of subgrid-scale convective processes and an explicit scheme for the treatment of grid-scale cloud water (ice) and rainwater (snow).
With higher resolution and improved condensation physics, the RFE model reproduces many detailed structures of the MCS, as compared with all available observations. In particular, the model predicts well the timing and location of the leading convective line followed by stratiform precipitation; the distribution of surface temperature and pressure perturbations (e.g., cold outflow boundaries, mesolows, mesohighs, and wake lows); and the circulation of front-to-rear flows at both upper and lower levels separated by a rear-to-front flow at midlevels.
Several sensitivity experiments are performed to examine the effects of varying initial conditions and model physics on the prediction of the squall system. It is found that both the moist convective adjustment and the Kuo schemes can reproduce the line structure of convective precipitation. However, these two schemes are unable to reproduce the internal flow structure of the squall system and the pertinent surface pressure and thermal perturbations. It is emphasized that as the grid resolution increases, reasonable treatments of both parameterized and grid-scale condensation processes are essential in obtaining realistic predictions of MCSs and associated quantitative precipitation.
Abstract
A one-dimensional (1D) version of a blowing snow model, called PIEKTUK-D, has been incorporated into a snow–sea ice coupled system. Blowing snow results in sublimation of approximately 12 mm of snow water equivalent (SWE), which is equal to approximately 6% of the annual precipitation over 324 days from 1997 to 1998. This effect leads to an average decrease of 9 cm in snow depth for an 11-month simulation of the Surface Heat Budget of the Arctic Ocean (SHEBA) dataset (from 31 October 1997 to 1 October 1998). Inclusion of blowing snow has a significant impact on snow evolution between February and June, during which it is responsible for a decrease in snow depth error by about 30%. Between November and January, however, other factors such as regional surface topography or horizontal wind transport may have had a greater influence on the evolution of the snowpack and sea ice. During these few months the new system does not perform as well, with a snow depth percentage error of 39%—much larger than the 12% error found between February and June. The results also indicate a slight increase of 4 cm on average for ice thickness, and a decrease of 0.4 K for the temperature at the snow–ice interface. One of the main effects of blowing snow is to shorten the duration of snow cover above sea ice by approximately 4 days and to lead to earlier ice melt by approximately 6 days. Blowing snow also has a very small impact on internal characteristics of the snowpack, such as grain size and density, leading to a weaker snowpack.
Abstract
A one-dimensional (1D) version of a blowing snow model, called PIEKTUK-D, has been incorporated into a snow–sea ice coupled system. Blowing snow results in sublimation of approximately 12 mm of snow water equivalent (SWE), which is equal to approximately 6% of the annual precipitation over 324 days from 1997 to 1998. This effect leads to an average decrease of 9 cm in snow depth for an 11-month simulation of the Surface Heat Budget of the Arctic Ocean (SHEBA) dataset (from 31 October 1997 to 1 October 1998). Inclusion of blowing snow has a significant impact on snow evolution between February and June, during which it is responsible for a decrease in snow depth error by about 30%. Between November and January, however, other factors such as regional surface topography or horizontal wind transport may have had a greater influence on the evolution of the snowpack and sea ice. During these few months the new system does not perform as well, with a snow depth percentage error of 39%—much larger than the 12% error found between February and June. The results also indicate a slight increase of 4 cm on average for ice thickness, and a decrease of 0.4 K for the temperature at the snow–ice interface. One of the main effects of blowing snow is to shorten the duration of snow cover above sea ice by approximately 4 days and to lead to earlier ice melt by approximately 6 days. Blowing snow also has a very small impact on internal characteristics of the snowpack, such as grain size and density, leading to a weaker snowpack.
Abstract
The Canadian urban and land surface external modeling system (known as urban GEM-SURF) has been developed to provide surface and near-surface meteorological variables to improve numerical weather prediction and to become a tool for environmental applications. The system is based on the Town Energy Balance model for the built-up covers and on the Interactions between the Surface, Biosphere, and Atmosphere land surface model for the natural covers. It is driven by coarse-resolution forecasts from the 15-km Canadian regional operational model. This new system was tested for a 120-m grid-size computational domain covering the Montreal metropolitan region from 1 May to 30 September 2008. The numerical results were first evaluated against local observations of the surface energy budgets, air temperature, and humidity taken at the Environmental Prediction in Canadian Cities (EPiCC) field experiment tower sites. As compared with the regional deterministic 15-km model, important improvements have been achieved with this system over urban and suburban sites. GEM-SURF’s ability to simulate the Montreal surface urban heat island was also investigated, and the radiative surface temperatures from this system and from two systems operational at the Meteorological Service of Canada were compared, that is, the 15-km regional deterministic model and the so-called limited-area model with 2.5-km grid size. Comparison of urban GEM-SURF outputs with remotely sensed observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) reveals relatively good agreement for urban and natural areas.
Abstract
The Canadian urban and land surface external modeling system (known as urban GEM-SURF) has been developed to provide surface and near-surface meteorological variables to improve numerical weather prediction and to become a tool for environmental applications. The system is based on the Town Energy Balance model for the built-up covers and on the Interactions between the Surface, Biosphere, and Atmosphere land surface model for the natural covers. It is driven by coarse-resolution forecasts from the 15-km Canadian regional operational model. This new system was tested for a 120-m grid-size computational domain covering the Montreal metropolitan region from 1 May to 30 September 2008. The numerical results were first evaluated against local observations of the surface energy budgets, air temperature, and humidity taken at the Environmental Prediction in Canadian Cities (EPiCC) field experiment tower sites. As compared with the regional deterministic 15-km model, important improvements have been achieved with this system over urban and suburban sites. GEM-SURF’s ability to simulate the Montreal surface urban heat island was also investigated, and the radiative surface temperatures from this system and from two systems operational at the Meteorological Service of Canada were compared, that is, the 15-km regional deterministic model and the so-called limited-area model with 2.5-km grid size. Comparison of urban GEM-SURF outputs with remotely sensed observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) reveals relatively good agreement for urban and natural areas.
Abstract
The role and impact that boundary layer and shallow cumulus clouds have on the medium-range forecast of a large-scale weather system is discussed in this study. A mesoscale version of the Global Environmental Multiscale (GEM) atmospheric model is used to produce a 5-day numerical forecast of a midlatitude large-scale weather system that occurred over the Pacific Ocean during February 2003. In this version of GEM, four different schemes are used to represent (i) boundary layer clouds (including stratus, stratocumulus, and small cumulus clouds), (ii) shallow cumulus clouds (overshooting cumulus), (iii) deep convection, and (iv) nonconvective clouds. Two of these schemes, that is, the so-called MoisTKE and Kuo Transient schemes for boundary layer and overshooting cumulus clouds, respectively, have been recently introduced in GEM and are described in more detail.
The results show that GEM, with this new cloud package, is able to represent the wide variety of clouds observed in association with the large-scale weather system. In particular, it is found that the Kuo Transient scheme is mostly responsible for the shallow/intermediate cumulus clouds in the rear portion of the large-scale system, whereas MoisTKE produces the low-level stratocumulus clouds ahead of the system. Several diagnostics for the rear portion of the system reveal that the role of MoisTKE is mainly to increase the vertical transport (diffusion) associated with the boundary layer clouds, while Kuo Transient is acting in a manner more consistent with convective stabilization. As a consequence, MoisTKE is not able to remove the low-level shallow cloud layer that is incorrectly produced by the GEM nonconvective condensation scheme. Kuo Transient, in contrast, led to a significant reduction of these nonconvective clouds, in better agreement with satellite observations. This improved representation of stratocumulus and cumulus clouds does not have a large impact on the overall sea level pressure patterns of the large-scale weather system. Precipitation in the rear portion of the system, however, is found to be smoother when MoisTKE is used, and significantly less when the Kuo Transient scheme is switched on.
Abstract
The role and impact that boundary layer and shallow cumulus clouds have on the medium-range forecast of a large-scale weather system is discussed in this study. A mesoscale version of the Global Environmental Multiscale (GEM) atmospheric model is used to produce a 5-day numerical forecast of a midlatitude large-scale weather system that occurred over the Pacific Ocean during February 2003. In this version of GEM, four different schemes are used to represent (i) boundary layer clouds (including stratus, stratocumulus, and small cumulus clouds), (ii) shallow cumulus clouds (overshooting cumulus), (iii) deep convection, and (iv) nonconvective clouds. Two of these schemes, that is, the so-called MoisTKE and Kuo Transient schemes for boundary layer and overshooting cumulus clouds, respectively, have been recently introduced in GEM and are described in more detail.
The results show that GEM, with this new cloud package, is able to represent the wide variety of clouds observed in association with the large-scale weather system. In particular, it is found that the Kuo Transient scheme is mostly responsible for the shallow/intermediate cumulus clouds in the rear portion of the large-scale system, whereas MoisTKE produces the low-level stratocumulus clouds ahead of the system. Several diagnostics for the rear portion of the system reveal that the role of MoisTKE is mainly to increase the vertical transport (diffusion) associated with the boundary layer clouds, while Kuo Transient is acting in a manner more consistent with convective stabilization. As a consequence, MoisTKE is not able to remove the low-level shallow cloud layer that is incorrectly produced by the GEM nonconvective condensation scheme. Kuo Transient, in contrast, led to a significant reduction of these nonconvective clouds, in better agreement with satellite observations. This improved representation of stratocumulus and cumulus clouds does not have a large impact on the overall sea level pressure patterns of the large-scale weather system. Precipitation in the rear portion of the system, however, is found to be smoother when MoisTKE is used, and significantly less when the Kuo Transient scheme is switched on.
Abstract
Numerical weather prediction is moving toward the representation of finescale processes such as the interactions between the sea-breeze flow and urban processes. This study investigates the ability and necessity of using kilometer- to subkilometer-scale numerical simulations with the Canadian urban modeling system over the complex urban coastal area of Vancouver, British Columbia, Canada, during a sea-breeze event. Observations over the densely urbanized areas, collected from the Environmental Prediction in Canadian Cities (EPiCC) network and from satellite imagery, are used to evaluate several aspects of the urban boundary layer features simulated in three model configurations with different grid spacings (2.5 km, 1 km, and 250 m). In agreement with the observations, results from the numerical experiments with 1-km and 250-m grid spacings suggest that two sea-breeze flows converge over the residential areas of Vancouver. The resulting convergence line oscillates around the hill ridge, depending on thermal contrast and flow strength. This propagation mode impacts the growing urban boundary layer, with the presence of subsidence and entrainment events. Urban-induced circulation is superimposed with the sea-breeze circulation and realistically slows down the propagation of the sea-breeze front to the south. A clear improvement is obtained for numerical experiments with 1-km instead of 2.5-km grid spacing. The use of subkilometer grid spacing provides a more detailed representation of the surface thermal forcing and of local circulations, with results more sensitive to the airflow variability and, thus, to the location of measurement sites. Joint analyses of kilometer- and subkilometer-scale numerical experiments are thus recommended for different environmental applications.
Abstract
Numerical weather prediction is moving toward the representation of finescale processes such as the interactions between the sea-breeze flow and urban processes. This study investigates the ability and necessity of using kilometer- to subkilometer-scale numerical simulations with the Canadian urban modeling system over the complex urban coastal area of Vancouver, British Columbia, Canada, during a sea-breeze event. Observations over the densely urbanized areas, collected from the Environmental Prediction in Canadian Cities (EPiCC) network and from satellite imagery, are used to evaluate several aspects of the urban boundary layer features simulated in three model configurations with different grid spacings (2.5 km, 1 km, and 250 m). In agreement with the observations, results from the numerical experiments with 1-km and 250-m grid spacings suggest that two sea-breeze flows converge over the residential areas of Vancouver. The resulting convergence line oscillates around the hill ridge, depending on thermal contrast and flow strength. This propagation mode impacts the growing urban boundary layer, with the presence of subsidence and entrainment events. Urban-induced circulation is superimposed with the sea-breeze circulation and realistically slows down the propagation of the sea-breeze front to the south. A clear improvement is obtained for numerical experiments with 1-km instead of 2.5-km grid spacing. The use of subkilometer grid spacing provides a more detailed representation of the surface thermal forcing and of local circulations, with results more sensitive to the airflow variability and, thus, to the location of measurement sites. Joint analyses of kilometer- and subkilometer-scale numerical experiments are thus recommended for different environmental applications.
Abstract
In this study, the ability of a turbulent kinetic energy (TKE)–based boundary layer scheme to reproduce the rapid evolution of the planetary boundary layer (PBL) observed during two clear convective days is examined together with the impact of including nonlocal features in the boundary layer scheme. The two cases are chosen from the Montreal-96 Experiment on Regional Mixing and Ozone (MERMOZ): one is characterized by strong buoyancy, a strong capping inversion, and weak vertical wind shear; the other displays moderate buoyancy, a weaker subsidence inversion, and significant wind shear near the PBL top. With the original local version of the turbulence scheme, the model reproduces the vertical structures and turbulent quantities observed in the well-developed boundary layer for the first case. For the second case, the model fails to reproduce the rapid evolution of the boundary layer even though the TKE and sensible heat fluxes are greatly overpredicted.
Some nonlocal aspects of the turbulence scheme are tested for these two cases. Inclusion of nonlocal (countergradient) terms in the vertical diffusivity equation has little impact on the simulated PBL. In contrast, alternative formulations of the turbulent length scales that follow the strategy proposed by Bougeault and Lacarrère have a greater influence. With the new turbulent lengths, entrainment at the top of the boundary layer is enhanced so that the depth of the well-mixed layer is much larger compared to that of the local simulations even though the turbulent sensible heat fluxes are smaller. Comparison with observations reveals, however, that the inclusion of these modifications does not improve all aspects of the simulation. To improve the performance and reduce somewhat the arbitrariness in the Bougeault–Lacarrère technique, a relationship between the two turbulent length scales (mixing and dissipation) used in the turbulence scheme is proposed. It is shown that, in addition to reducing the sensitivity of the results to the particular formulations, the simulated boundary layer agrees better with observations.
Abstract
In this study, the ability of a turbulent kinetic energy (TKE)–based boundary layer scheme to reproduce the rapid evolution of the planetary boundary layer (PBL) observed during two clear convective days is examined together with the impact of including nonlocal features in the boundary layer scheme. The two cases are chosen from the Montreal-96 Experiment on Regional Mixing and Ozone (MERMOZ): one is characterized by strong buoyancy, a strong capping inversion, and weak vertical wind shear; the other displays moderate buoyancy, a weaker subsidence inversion, and significant wind shear near the PBL top. With the original local version of the turbulence scheme, the model reproduces the vertical structures and turbulent quantities observed in the well-developed boundary layer for the first case. For the second case, the model fails to reproduce the rapid evolution of the boundary layer even though the TKE and sensible heat fluxes are greatly overpredicted.
Some nonlocal aspects of the turbulence scheme are tested for these two cases. Inclusion of nonlocal (countergradient) terms in the vertical diffusivity equation has little impact on the simulated PBL. In contrast, alternative formulations of the turbulent length scales that follow the strategy proposed by Bougeault and Lacarrère have a greater influence. With the new turbulent lengths, entrainment at the top of the boundary layer is enhanced so that the depth of the well-mixed layer is much larger compared to that of the local simulations even though the turbulent sensible heat fluxes are smaller. Comparison with observations reveals, however, that the inclusion of these modifications does not improve all aspects of the simulation. To improve the performance and reduce somewhat the arbitrariness in the Bougeault–Lacarrère technique, a relationship between the two turbulent length scales (mixing and dissipation) used in the turbulence scheme is proposed. It is shown that, in addition to reducing the sensitivity of the results to the particular formulations, the simulated boundary layer agrees better with observations.
Abstract
The objective and subjective evaluations that led to the implementation of the Fritsch and Chappell (FC) convective scheme in the new 24-km Canadian operational regional model are described in this study. Objective precipitation scores computed for a series of 12 benchmark cases equally distributed throughout all seasons and for a parallel preimplementation run of the new version of the model during summer 1998 show the positive impact of increasing the horizontal resolution and of including the FC scheme (instead of the Kuo scheme used in the previous version of the operational model). The comparison is particularly in favor of the FC configuration for the summertime parallel preimplementation run, with improved biases and threat scores, while it is nearly neutral for the 12 benchmark cases comprised mostly of large-scale weather systems.
Examination of a summertime case study confirms the superiority of FC over Kuo for the numerical representation of the structure and evolution of mesoscale convective systems. A wintertime case study, on the other hand, reveals that precipitation patterns with the two model configurations are quite similar, even though the FC scheme is essentially inactive for weather systems organized on such large scales. In contrast with the Kuo simulation, most of the precipitation occurs on the grid scale when using FC. This different partitioning of precipitation into implicit and explicit components is more consistent with the mesoscale-resolving capabilities of the model. It is also observed that the new model physics gives rise to more realistic deepening of coastal large-scale depressions.
The different implicit/explicit partitioning for Kuo and FC is clearly exposed with precipitation statistics from the 12 benchmark cases. With Kuo, it is found that implicit precipitation is produced over areas as large as (and even larger than) that associated with grid-scale precipitation; it is also shown that with this configuration most of the precipitation occurs at weak rates and is mainly produced by the implicit scheme. The results with FC are more realistic, in the sense that convective precipitation only covers a small fraction of the model domain (i.e., 1%–2%) and that both precipitation schemes are dominant in their respective areas, that is, weak precipitation for the explicit scheme and more intense precipitation for the implicit scheme.
Abstract
The objective and subjective evaluations that led to the implementation of the Fritsch and Chappell (FC) convective scheme in the new 24-km Canadian operational regional model are described in this study. Objective precipitation scores computed for a series of 12 benchmark cases equally distributed throughout all seasons and for a parallel preimplementation run of the new version of the model during summer 1998 show the positive impact of increasing the horizontal resolution and of including the FC scheme (instead of the Kuo scheme used in the previous version of the operational model). The comparison is particularly in favor of the FC configuration for the summertime parallel preimplementation run, with improved biases and threat scores, while it is nearly neutral for the 12 benchmark cases comprised mostly of large-scale weather systems.
Examination of a summertime case study confirms the superiority of FC over Kuo for the numerical representation of the structure and evolution of mesoscale convective systems. A wintertime case study, on the other hand, reveals that precipitation patterns with the two model configurations are quite similar, even though the FC scheme is essentially inactive for weather systems organized on such large scales. In contrast with the Kuo simulation, most of the precipitation occurs on the grid scale when using FC. This different partitioning of precipitation into implicit and explicit components is more consistent with the mesoscale-resolving capabilities of the model. It is also observed that the new model physics gives rise to more realistic deepening of coastal large-scale depressions.
The different implicit/explicit partitioning for Kuo and FC is clearly exposed with precipitation statistics from the 12 benchmark cases. With Kuo, it is found that implicit precipitation is produced over areas as large as (and even larger than) that associated with grid-scale precipitation; it is also shown that with this configuration most of the precipitation occurs at weak rates and is mainly produced by the implicit scheme. The results with FC are more realistic, in the sense that convective precipitation only covers a small fraction of the model domain (i.e., 1%–2%) and that both precipitation schemes are dominant in their respective areas, that is, weak precipitation for the explicit scheme and more intense precipitation for the implicit scheme.
